1. Field of the Invention
The present invention relates to magnetic recording media. In particular, the present invention relates to perpendicular magnetic recording media having a magnetically soft underlayer and a hard magnetic recording layer that is disposed over the soft underlayer.
2. Description of Related Art
Thin film magnetic recording media are composed of multiple layers, including one or more magnetic recording layers, disposed on a substrate. Typically, the magnetic recording layer includes small magnetic grains that have an easy magnetization axis that is magnetically oriented longitudinally (i.e., in plane) with respect to the magnetic layer.
The areal density of such longitudinal magnetic recording media has been increasing at a compounded growth rate of about 60% per year and areal densities as high as 100 gigabits per square inch (Gbit/in2) have been demonstrated. Scaling longitudinal recording media to higher areal densities requires smaller magnetic grains. However, as the grain size is reduced, thermal fluctuations can cause the magnetic domains to “flip”, resulting in a loss of magnetization over a period of time. Media having a higher magnetic coercivity (Hc) and an increased track density (tracks per inch, or TPI) can mitigate this problem. However, the large write head fields that are needed for good overwrite of high coercivity media can lead to excessive fringing, negatively affecting the data written on adjacent tracks.
Perpendicular (i.e., vertical) magnetic recording media have been proposed as a way to increase areal densities beyond 100 Gbit/in2. Perpendicular magnetic recording media include a magnetic recording layer having an easy magnetization axis that is oriented substantially perpendicular to the magnetic layer. A perpendicular write-head, such as a monopole write-head or a shielded pole write-head is utilized to magnetize the grains in the perpendicular recording layer.
More specifically, the write-head for perpendicular recording media includes a write pole and a return pole that is magnetically coupled to the write pole. An electrically conductive magnetizing coil surrounds the yoke of the write pole and is adapted to switch the polarity of the magnetic field applied to the write pole. During operation, the recording head flies above the magnetic recording medium by a distance referred to as the fly height, and an electrical current is passed through the coil to create a magnetic flux within the write pole. The magnetic flux passes from the write pole tip through the magnetic recording layer and into a magnetically soft underlayer (SUL) disposed beneath the magnetic recording layer. The SUL causes the magnetic flux to pass across to the return pole of the write-head. In addition, the SUL produces magnetic charge images during read operations, increasing the magnetic flux and the playback signal. Examples of such perpendicular magnetic recording heads and associated perpendicular recording media are disclosed in: U.S. Pat. No. 4,423,450 by Hamilton; U.S. Pat. No. 4,656,546 by Mallary; and U.S. Pat. No. 4,748,525 by Perlov. Each of these U.S. patents is incorporated herein by reference in its entirety. Perpendicular recording media can support higher areal densities than conventional longitudinal media, in part due to reduced demagnetizing fields in the recording transitions (i.e., the transition from one magnetic bit to the next).
One design for magnetic media incorporating a perpendicular magnetic recording layer is illustrated in FIG. 1. Disposed above the perpendicular magnetic recording media 100 is a read-write head 110, which in operation flies above the perpendicular magnetic recording layer 106. The read-write head 110 includes a write element 112 having a magnetic coil 113 for applying electrical current to the write element 112. The read-write head 110 also includes a reading element 114 and a read shield 116 for reading data from the perpendicular magnetic recording media.
The recording media 100 includes two magnetic layers disposed on a substrate 102. The first layer is a relatively thick SUL 104 that serves as a return path for the magnetic field generated by the write element 112. The SUL 104 is typically about 200 nm thick, but can be thinner with accompanying changes in the read-write head design. The SUL 104 increases the write field and provides the added benefit of increasing the signal strength by imaging the perpendicular magnetic recording layer 106. The SUL is magnetically soft and the magnetic permeability of the SUL 104 should be at least about 100 to enable the large write fields that are necessary to write on the high coercivity perpendicular magnetic recording layer 106. It is generally preferred to have the anisotropy of the SUL 104 directed along the radial (cross-track) direction, although it can alternatively be along the circumferential (down-track) direction. The perpendicular magnetic recording layer 106 is a magnetically hard layer that has high coercivity, a negative nucleation field and perpendicular anisotropy. The magnetic recording media 100 can also include a non-magnetic spacer layer 108 between the two magnetic layers.
Many magnetic alloy systems have been utilized for the perpendicular magnetic recording layer 106, including the CoCrPt alloys that are traditionally used in longitudinal media, CoPt or CoPd multilayers, and rare-earth and transition metal alloys that are traditionally employed in magneto-optic drives. The most successful alloy system with respect to perpendicular recording performance has been the CoCrPt-oxide system. High coercivity, a squareness=1, negative nucleation, small grains, and magnetic isolation can all be simultaneously achieved using this oxide-containing alloy. A Ru intermediate layer and a Ta seed layer can also be used, resulting in media with good thermal stability and excellent recording performance. See, for example, U.S. Patent Publication No. 2003/0091798 by Zheng et al., which is incorporated herein by reference in its entirety.
One factor that is impeding the implementation of perpendicular recording media is the media noise resulting from the SUL. The noise can be due to fields generated at the magnetic domain walls (spike noise) or can be due to magnetic charges arising from external fields that are sensed by the read head. Since the domain walls can move after repeated writing operations, the noise sources are not fixed on the disk surface. Therefore, the distribution of magnetic domains within the SUL must be carefully controlled.
To eliminate domain walls and their associated spike noise, a layered SUL can be utilized. For example, layers of the magnetic alloys CoZrTa or FeTaC having a thickness of from about 30 nm to about 40 nm can be interleaved with layers of C or Ta having a thickness of about 1 nm to about 2 nm. However, the layered SUL will have a reduced magnetic permeability. Further, for a total SUL thickness above about 100 nm, the layered SUL requires additional process steps, which increases manufacturing costs.
Modifications to the SUL have been proposed to improve the magnetic properties of the media. For example, U.S. Patent Application Publication No. 2002/0028357 by Shukh et al. discloses a perpendicular magnetic recording medium with anti-ferromagnetic coupling in the SUL. The SUL is a laminated structure that includes first and second soft magnetic layers; first and second interface layers and a non-magnetic coupling layer between the first interface layer and the second interface layer. The soft magnetic layers and the interface layers are both anti-ferromagnetically exchange coupled to one another through the non-magnetic coupling layer such that their magnetizations are oriented anti-parallel to one another. It is disclosed that exchange anti-ferromagnetic coupling forms a strong in-plane bias field that maintains the ferromagnetic coupled layers in a mostly single domain state.
U.S. Patent Application Publication No. 2003/0022023 by Carey et al. discloses dual-layered perpendicular magnetic recording media. The magnetic recording media includes a laminated SUL structure that has at least two ferromagnetic film layers that are exchange-coupled across an anti-ferromagnetic coupling layer. The magnetic moments of the ferromagnetic film layers are oriented anti-parallel.
Multi-domains in the SUL can also be suppressed by applying an in-plane (longitudinal) bias field during fabrication. For example, the SUL can be magnetically “pinned” by an anti-ferromagnetic layer with in-plane (longitudinal) anisotropy that is disposed beneath the SUL, as is disclosed in U.S. Patent Application Publication No. 2001/0038932 by Uwazumi. The SUL can also be pinned by a hard magnetic layer with in-plane anisotropy, as is disclosed in U.S. Patent Application Publication No. 2002/0136930 by Oikawa et al. According to Oikawa et al., the bias field increases the effective anisotropy field of the SUL. However, the bias field also leads to increased coercivity of the SUL.
The use of anti-ferromagnetic materials usually requires a high temperature annealing step in the presence of a magnetic field to set the anisotropy along a particular direction, or requires a magnet array fitted to a sputtering unit that produces a complex radial field pattern. For both cases, setting the bias field requires a special magnet or electromagnet arrangement, which increases cost and reduces throughput. For permanent magnet pinning, the fields required to set the magnetization direction can also be very large, adding to the difficulty of longitudinal bias. Moreover, for presently known magnetic systems, the anisotropy direction cannot be fabricated along the radial direction, but is random in the plane. To some extent, the anisotropy direction can be aligned along the circumferential direction, but cannot be completely aligned.
There is a need for a perpendicular recording medium where the foregoing limitations and disadvantages are reduced or eliminated.